Dovetails Measuring Calculator

Instructions: Enter your known values here (excluding either S Male or S Female, if unknown), then skip to the bottom and click the Go button.

Enter the S Female (top red line) Measurement:

Enter the inner distance between the two dowels here. You can use a digital caliper for this or you could use
Adjustable Parallels,
which you would lock into place between the dowels, and then measure with a
micrometer.
If you don't have one of the parts, for example if you don't have the female part, but you want to make it by taking measurements off the male part, just leave the female S value blank. Once you get the male Y and male T values, you can
use them to figure some reasonable values for the female Y and T values. All you have to do is make the female Y a bit larger than the male T and make the female T a bit larger than the male Y. How much larger is up to you, depending on
how tight a fit you're looking for. Leave this blank for now if you don't have the missing part, and then use the Target S button to figure out what you need the S value to be.

Enter the diameter (D) (green line) of the dowels:

Enter the diameter of the dowels here. I'm calling them dowels, but they can be anything cylindrical shaped,
such as dowels, pins, pipes, drill bits, so long as they have a known diameter and come reasonably close in size as to match the scale as depicted.

Enter the angle (usually 60 degrees) of the dovetails:

Enter the angle of the dovetails here. These are usually 60 degrees, but you might find some 45 degree ones, too.

Enter the S Male (bottom red line) Measurement:

Enter the outer distance between the two dowels here (outside to outside). If you don't have one of the parts, for example if you don't have the male part,
but you want to make it by taking measurements off the female part, just leave the male S value blank for now. Once you get the female Y and female T values,
you can use them to figure some reasonable values for the male Y and T values. All you have to do is make the male Y a bit smaller than the female T
and make the male T a bit smaller than the female Y. How much smaller is up to you, depending on how tight a fit you're looking for. Leave this blank
for now if you don't have the missing part, and then use the Target S button to figure out what you need the S value to be.

What are dovetails and why would I want to measure them? Dovetails are types of joints used to connect two pieces (such as wood or metal) together for a (usually) tighter fit. Often they are used where the two pieces must slide relative to one another
in only one of the 3 axes (x, y, and z). For example, you might see a dovetail joint used on a
vise so the movable jaw can only move either towards or away from the fixed jaw, but in no other direction along any other axis. In some machinery when it is expected to be put into heavy use, small spacers (called gibs) are often placed inside the
dovetail, the purpose being to allow the joint to be tightened (by adjusting the gibs) periodically as the parts wear. Of course, the reason you would want to measure dovetails would be preparatory to cutting them.

But why would I need a calculator to make some measurments? Well, you could certainly measure them without this calculator, but where great precision is needed they can be difficult to directly measure accurately enough. For example, if a machinist is
making an accessory for his metal lathe that will extend an existing dovetail, he would need some very accurate measurements of the existing dovetail in order to fashion a piece that would mate with it properly. The primary difficulty is in getting
the measuring device into the corners. No matter how tiny the measuring tip of the instrument, it will still never quite get into the corner all the way. (Note the circles in the images above, and how they don't reach completely into the corners.)
It's far easier to get really good measurements of the red lines in the images above than it is to do the same with the white lines. By using dowels (or other similarly shaped objects) the dovetails can be measured with great accuracy, provided
we know with great accuracy the diameter of the dowels and the angles of the dovetails, along with a little bit of trigonometry. This calculator handles all the trig for you.

In the top image you see a female dovetail, the bottom is the male. One of the pieces must be flipped over in order for the parts to be mated and the joint formed. The white lines and the blue lines color code the matching surfaces. The 2 circles represent
the steel dowels used to aid in the measurements. The dowels are not a part of the joint when the pieces are put together. The green line represents the diameter of the dowel. I say dowels, but these can be any rod-shaped objects of any diameter
reasonably close enough to match the scale as shown in the images. The yellow line represents the height of the dovetail. It should be the same for both pieces, but if it's different, use the male part's height. (The Y parts of the tricky ones,
but you should be able to get decent measurements for the T parts, both male and female, which can be used as a crosscheck for the values calculated with this calculator. If the T values the calculator gives you aren't very close to the ones you're
directly measuring, then something isn't right somewhere, so don't just accept these numbers blindly.)

There might come a time when you only have one part and you want to make a matching other part. For example, you might only have the male part and you want to make a matching female part. If that's the case, just leave the S Female box empty. (Or leave
the S Male box empty if that's the part you don't have.) All you have to do is remember the T Female measurement needs to be a little bit bigger than the Y Male part and the Y Female needs to be a little bit bigger than the T Male. The male part
has to be smaller in order to fit inside the female part. How much bigger or smaller to make the missing part is up to you, depending on how tight a fit you need. Here's where the Target S Female and Target S Male buttons come in. Enter your targeted
value you want to hit for the Male or Female T measurement (whichever one you don't already have), and then hit the Target S Female or Target S Male button. The calculator will figure out the S value for you based on the T value you entered as
your target. Once we have an S value to work with the calculator will go ahead and calculate the missing Y value for you, based on the height, diameter, and T target value you just entered. You can then use the S value as a reference as you cut
and measure using the dowel method while the work is on the machine.

If you want to watch a great youtube video on this process, see this one by mrpete222, who also has a great many other fantastic videos if you're interested in machine shop-related videos.

--Mark Ganson

Send any questions/comments/flames to mwganson at hotmail dot com with dovetails in the subject line.